In ALD and CVD deposition processes, often reactants are used that have a very low vapor pressure. First of all this is a problem as it becomes difficult to transport the reactant into the reactor. Furthermore, after the reactant exposure step it will be difficult to remove the reactant from the reaction chamber by purging of evacuation.
Slow purging of low vapor pressure reactants is particularly problematic for an ALD process where repeated and alternating pulses of at least two reactants are used and where it is important that the reactants remain separated. Exemplary processes are deposition processes for metal oxides which are going to be used as a high-k gate oxide material in MOSFET structures. ALD is a preferred technique to deposit films in a controllable manner by sequential and alternating pulses of at least two mutually reactive reactants. Metal halides are suitable metal source chemicals for ALD as they can easily be produced and are thermally stable and they tend to react strongly with water vapor at low temperatures. For HfO2, a frequently used high-k material, the best material properties are obtained with HfCl4+H2O as reactants, as compared to processes using other Hf-containing source materials such as metal-organic Hf compounds. The main drawback of many metal chlorides is their relatively low vapor pressure. Usually source temperatures around 150-200° C. are required to create sufficient vapor pressure for transportation of the reactant from the source container to the reactor. Even at these temperatures the vapor pressure is relatively low. This makes the reactor design very challenging and the removal of HfCl4 by purging and/or evacuation difficult.
The vapor pressure of molecules is affected by several different factors, such as: i) the molecular weight; ii) tendency to polymerization; and iii) intermolecular bonds. HfCl4 has a particularly low vapor pressure, particularly when its molecular weight is compared to other precursors. The molecular weight of HfCl4 is 320.5 g/mol and its vapor pressure is only 1 torr at 190° C. By way of comparison, the molecular weight of WF6 is similar to that of HfCl4, 297.8 g/mol, but WF6 has a much larger vapor pressure of 860 Torr at 21° C. The reason is that HfCl4 has a non-saturated coordination. Hafnium is a relative large metal and thus most of its compounds have a high coordination number. The most usual coordination number for Hf is eight [Chemistry of the Elements, Greenwood, N. N.; Earnshaw, A; © 1997 Elsevier]. Monomeric HfCl4 would have a coordination number of four. This is too low for hafnium and thus it will tend to make coordination bonds to other HfCl4 molecules so that the coordination sphere gets saturated. This results in a dramatic reduction in vapor pressure.
It is possible that the vapor pressure of a reactant is high but nevertheless it has a non-saturated coordination. Examples are molecules having a lone pair of electrons, such as H2O and NH3. These molecules also have a strong tendency to increase their coordination number and therefore they have a strong tendency to stick to the reactor walls. Additionally, reaction by-products generated in the film deposition process might have a non-saturated coordination and corresponding tendency to stick to the reactor wall.
Consequently, in film deposition processes using vapor phase reactants with a non-saturated coordination, or generating reaction by-products with a non-saturated coordination there is a need for a method to prevent non-saturated molecules from forming coordinating bonds by sticking to the reactor walls or by forming bonds with other molecules of the same kind to increase their coordination number.